A known transmission system includes redundant transmitters, receivers, and transmission paths in preparation for a failure of devices, transmission paths, or so on. One of redundant transmission systems is referred to as a work system and the other is referred to as a standby system. The work system is used for communication services at normal time and the standby system is used for communication services in place of the work system if the work system fails. The function for switching from the work system to the standby system is referred to as the automatic protection switch (APS).
For example, Japanese National Publication of International Patent Application No. 2007-515815 discloses an optical transmission system in which two optical input units and two optical output units disposed in a transmission apparatus are connected to a client apparatus via a Y-shaped cable.